-LRB- CNN -RRB- -- The U.S. Army Corps of Engineers could open the Morganza Spillway as early as Saturday , Louisiana Gov. Bobby Jindal said Friday .

The Mississippi River Commission has directed the Corps to operate the crucial spillway once river flows reach a certain trigger : 1.5 million cubic feet per second . Projections indicate the tipping point could be hit as early as Saturday evening , Jindal has said .

Opening the spillway would lower anticipated cresting levels along the lower Mississippi River and divert water from Baton Rouge and New Orleans but would flood much of low-lying south-central Louisiana . Seven parishes are expected to be affected by the opening , according to the Corps .

The Mississippi River Commission has advised a `` slow opening , '' and the flood would spread gradually over several days , the Corps said .

The Morganza Spillway has not been opened since 1973 .

Louisiana state and local officials braced for the possibility of major flooding in the Atchafalaya River Basin if , or when , federal authorities open the spillway north of Baton Rouge . They advised residents to expect road closings .

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The National Guard worked around the clock to construct a flood barrier in Morgan City , Louisiana , where the Atchafalaya River was already 3.15 feet above flood stage , according to the National Weather Service .

The strategy in Morgan City , officials say , is to reinforce the levees around the city . That 's where efforts were being focused Friday , rather than on handing out sandbags to individual residents .

`` Really , we 're just waiting , '' said Evie Bertaut , who has lived in Morgan City for 50 years .

Officials believe that the levees will protect the city from flooding , but some are taking preliminary precautions , she said . At Sacred Heart Church , where Bertaut works , people spent the day moving important documents such a baptismal , marriage and financial records to the second floor .

`` Most people are getting their photographs together , things that you ca n't replace in case you have to go , '' she said .

A collective gasp as Louisiana town braces for flood

Meanwhile , in the Arkansas town of Helena , the river crested at 56.5 feet -- 12.5 feet above flood stage , according to the National Weather Service .

The river 's slow pace has given emergency responders more time to prepare , forecasters said . But while the slow-moving water gives residents extra time to get ready , it also means that land could remain under water for some time .

Jindal urged southeastern Louisiana residents to evacuate .

`` Now is the time to take action , '' he said .

The U.S. Coast Guard said floodwaters could close the Mississippi River to ships at the New Orleans port as early as Monday morning .

To help New Orleans , the Army Corps of Engineers said Friday that it will open 52 more bays at the Bonnet Carre Spillway just north of the city , diverting water into Lake Pontchartrain . That will mean a total of 264 bays will be open in the 350-bay spillway .

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The National Weather Service said that as of Friday morning , the river was at 16.8 feet in New Orleans , just a fraction below flood stage . It is expected to crest May 23 at more than 19 feet . The New Orleans levees are built to withstand 20 feet , according to the weather service .

Upriver in Vicksburg , Mississippi , Police Chief Walter Armstrong said 600 residents had been evacuated as of Thursday night . The river was expected to crest at 57.5 feet . Flood stage at Vicksburg , the level at which the river may begin flowing over its banks , is 43 feet .

Armstrong said he expected higher water Friday , with more homes affected . More than two dozen roads were closed and about 45 businesses will be closed by Friday .

Homes that were built between the levee and the Mississippi River were the first affected .

`` We estimate that every home built on the river side of the levee from Memphis all the way to the Louisiana line is flooded , '' said Mike Womack , executive director of the Mississippi Emergency Management Agency .

Residents near Vicksburg counted on a levee for protection . In addition to the mainline levee along the river , starting near Vicksburg and extending northeast for more than 20 miles , a so-called backwater levee offers shelter .

The backwater levee is designed to keep water from backing into the Yazoo River delta and is designed lower than the mainline levee so that water can flow over it . That level is expected to be reached Monday , said Charlie Tindall , attorney for the Board of Mississippi Levee Commissioners .

The backwater levee was being `` armored '' by a heavy plastic coating to prevent it from washing out , he said .

Nonetheless , 1.4 million acres in Mississippi , including 602,000 acres where crops are growing , could flood , said Rickey Grey of the state 's Department of Agriculture .

Across the South and lower Midwest , floodwaters have covered about 3 million acres of farmland , eroding for many farmers what could have been a profitable year for corn , wheat , rice and cotton , officials said .

In Arkansas , the Farm Bureau estimated that damage to the state 's agriculture could top more than $ 500 million as more than 1 million acres of cropland are under water .

Womack talks about flood costs

`` It 's in about 10 feet of water , '' Dyersburg , Tennessee , farmer Jimmy Moody said of his 440 acres of winter wheat , which was to be harvested in the coming month .

Other farmers in Mississippi , Missouri , Tennessee and Arkansas rushed to salvage what wheat they could ahead of the rising water . As for corn , farmers who were able to get into the fields during a soggy planting season in late March and April are seeing their crops in some cases under several feet of water .

Some officials said Thursday that spillover effects resulting from the flood could threaten other industries . That includes the possibility that the Waterford 3 nuclear power plant in Taft , Louisiana , could be closed , according to CNN affiliate WGNO .

The Mississippi River is expected to crest at 26.6 feet in Taft on May 23 . If it reaches 27 feet , officials told WGNO , the plant 's water intake system could shut down .

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Carl Rhode of Entergy , the plant 's operator , told WGNO that the threat to the intake system is not a matter of nuclear safety .

However , Scott Welchel , a St. Charles Parish Emergency Operations Center official , said shutting down the plant would have a `` domino effect '' on local industries .

`` It would impact every industry along the river , '' Welchel said . `` That 's just something that is n't easy for people to deal with , especially on a moment 's notice . ''

For residents in communities along the river , the damage has been far more devastating than can be measured in dollars and cents .

Danny Moore of Millington , Tennessee , told CNN affiliate WPTY that the recent disaster marked the second time in one year that flooding took away nearly everything he had .

Moore said that after a flood destroyed all of his furniture last year , he decided to move everything he owned into rented storage space . However , those belongings were destroyed when his storage unit was flooded several days ago .

`` They say bad luck comes in threes . I hope this is the end of it , '' Moore told WPTY .

The Millington resident said he lost a house to a fire in 2009 . Moore said he is too preoccupied with taking care of his girlfriend , who is suffering from an infection that is damaging her liver , to look for new furniture .

`` We 'll do what we 've got to do and keep praying , '' Moore said , holding back tears .

CNN 's Mariano Castillo , Mia Aquino and Erica Henry contributed to this report .

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NEW : Army Corps of Engineers is expected to open the spillway once river flows hit a trigger

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NEW : Projections indicate the tipping point could come as early as Saturday , Jindal says

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NEW : The Morganza Spillway has not been opened since 1973

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Opening the Morganza Spillway would flood the Atchafalaya River Basin